The move makes some sense to me. While the red zone offense was better, Romo's YPA was much lower than previous years (See Bradshaw's rant earlier in the early in the year). So if you want more aggressive passing game and don't want Garrett to go back to play calling, then you have to hire someone else. If you don't want to mess with the O-line then Callihan has to stay too.
That means you hire a guy who will call the passing game. The Lions have had an aggressive passing game that will fit Romo's ability to get the ball to receivers down field. I think the move might quietly improve things.